Emission analyzer

ABSTRACT

After a switching element  13  is turned on, a charge controller  16  determines whether or not a current detected by an excitation current detector  15  has reached a predetermined level, and turns off the switching element  13  if the predetermined level has been reached. When the excitation current is controlled to a constant level, the excitation energy stored in a flyback transformer  12  also becomes constant, and this constant energy is stored in a capacitor  22  every time the switching element  13  is turned off. The charge control section  16  repeats the on/off operation of the switching element  13  a predetermined number of times at predetermined intervals of time before discontinuing the charging operation. Consequently, a constant amount of energy is held in the capacitor  22  when the charging operation is discontinued, and the period of time from the beginning to the completion of charging also becomes constant. Thus, the same conditions are constantly created while the spark discharge is repeated, so that the accuracy and reproducibility of the analysis are enhanced.

TECHNICAL FIELD

The present invention relates to an optical emission analyzer utilizinga spark discharge.

BACKGROUND ART

In a spark discharge emission analyzer, an amount of energy stored in acapacitor is supplied to a discharge electrode to generate a sparkdischarge between the electrode and a metallic sample, whereby the atomsof the elements contained in the metallic sample are vaporized, and thevaporized atoms are excited by a discharge plasma. Since each atomexcited in the plasma emits light at a wavelength characteristic of theelement, it is possible to determine the quantity of the element bydispersing the emitted light into a spectrum and measuring the lightintensity at the aforementioned wavelength. It is also possible toperform a qualitative analysis of an unknown element contained in thesample by creating an emission spectrum with a predetermined wavelengthrange and searching for a wavelength at which a line spectrum ispresent. Normally, the spark discharge is repeated at a frequency from afew tens to several hundreds of hertz, and the photometrical valuesobtained for each discharge are integrated to improve the measurementaccuracy.

In this type of emission analyzer, it is necessary to charge thecapacitor to a voltage level of several hundreds of volts within arelatively short period of time to generate a spark discharge. For thispurpose, a switching type capacitor-discharging circuit has been widelyused in recent years (for example, refer to Patent Document 1). FIG. 3is a block diagram showing the configuration of an emission analyzerusing a conventional switching type capacitor-charging circuit.

In this emission analyzer, the emitting section consists of acapacitor-charging circuit 1, a capacitor circuit 2, an igniter circuit3, and an emission stand 4. The capacitor-charging circuit 1 includes adirect-current (DC) power source 11, a flyback transformer 12 withprimary and secondary windings, a switching element 13 such as afield-effect transistor (FET), and a charge controller 14 for drivingthe switching element 13. The capacitor circuit includes a rectifyingdiode 21, a discharge capacitor 22 for storing electrical energy to beused for generating a discharge, and a charged voltage detector 23 fordetecting the charged voltage of the discharge capacitor 22. The ignitercircuit 3 includes an igniter transformer 31 with primary and secondarywindings, and an igniter driver 32 for generating a high level ofvoltage in the secondary winding of the igniter transformer 31. Theemission stand 4 includes a discharge electrode 41 and a sample 42 to bemeasured, which is typically a piece of metal.

In the capacitor-charging circuit 1, the primary winding of the flybacktransformer 12 and the switching element 13 are serially connected toboth ends of the DC power source 11, respectively. When the switchingelement 13 is turned on (i.e. made to be conductive) by the chargecontroller 14, a DC current is supplied to the primary winding of theflyback transformer 12, whereby an excitation energy is stored in theflyback transformer 12. The charge controller 14 maintains the switchingelement 13 in the “on” state for a predetermined period of time.Subsequently, when the switching element 13 is turned off, a counterelectromotive force arises in the secondary winding of the flybacktransformer 12. As a result, the excitation energy that has beenaccumulated in the flyback transformer 12 is supplied through therectifying diode 21 into the discharge capacitor 22 in the capacitorcircuit 2. Thus, the discharge capacitor 22 is charged.

The on/off state of the switching element 13 is controlled as shown inFIG. 4. Every time the switching element 13 is turned off, the chargedvoltage of the discharge capacitor 22 increases in a stepwise manner dueto the excitation energy that has been accumulated in the flybacktransformer 12. The charged voltage detector 23 monitors the chargedvoltage of the discharge capacitor 22. Based on this monitored value,the charge controller 14 determines whether or not the charged voltagehas exceeded a predetermined level V1. The charge controller 14 repeatsthe on/off controlling of the switching element 13 until the chargedvoltage exceeds the predetermined level V1. When the charged voltage hasexceeded the predetermined voltage V1, the charge controller 14 stopsturning on the switching element 13 to discontinue the charging of thedischarge capacitor 22.

After the charging operation is completed in this manner, the igniterdriver 32 in the igniter circuit 3 generates a high voltage in theigniter transformer 31, whereupon a spark discharge occurs between thedischarge electrode 41 and the metallic sample 42. This makes thesurface of the metallic sample 42 locally heated, vaporizing the atomsof an element present on the sample surface. Simultaneously, the energystored in the discharge capacitor 22 is transferred into the gap betweenthe discharge electrode 41 and the metallic sample 42 to create aplasma, in which the vaporized atoms are excited by electrons. When anatom returns from the excited state to a stable state, it emits lighthaving a wavelength corresponding to the energy difference between thetwo states. The photometric section 5, which includes a light-dispersingelement, photodetector and other components, measures the emitted lighthaving a wavelength characteristic of the element to collect informationrelating to the elements contained in the metallic sample 42.

As just described, the capacitor-charging circuit 1 based on theconventional switching method accumulates a required amount ofelectrical energy in the discharge capacitor 22 by discontinuing thecharging operation when the charged voltage of the discharge capacitor22 has been found to be higher than the predetermined level V1.

However, the previously described capacitor-charging circuit 1 has thefollowing problem: Since the period of time during which the switchingelement 13 is in the “on” state is definitely set, the amount ofexcitation energy that is accumulated within each on/off cycle of theswitching element 13 changes if the voltage of the DC power source 11changes. Furthermore, the capacitance of the discharge capacitor 22varies due to, for example, a temperature change of the capacitor. Sucha change in the amount of excitation energy accumulated in the flybacktransformer 12 or the capacitance of the discharge capacitor 22 leads toa change in the number of on/off operations of the switching element 13necessary for charging the discharge capacitor 22 to the constantvoltage V1. Then, as shown in FIG. 5, the point in time at which thecharged voltage of the discharge capacitor 22 exceeds the thresholdlevel V1 will vary, which means that the charged voltage of thedischarge capacitor 22 at the moment of discontinuing the chargingoperation can change by up to an amount corresponding to one on/offcycle of the switching element 13. Thus, the amount of energy to beaccumulated in the discharge capacitor 22 changes.

Even if the charged voltage eventually reaches the same level, if thecapacitance of the discharge capacitor 22 changes due to theaforementioned reason, the amount of energy held in the dischargecapacitor 22 will change since the discontinuation of the chargingoperation is controlled based on the monitored value of the chargedvoltage of the same capacitor 22.

If the energy stored in the discharge capacitor 22 at the moment ofgenerating the spark discharge is not constant, the state of the plasmaat the moment of discharging will change. Therefore, even if the elementcontent of the metallic sample 42 is the same, the emission intensitywill vary, which may possibly deteriorate the accuracy orreproducibility of the analysis.

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 2004-333323 (Paragraphs 0004-0007; FIG. 5)

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

Thus, although it is essential for a spark discharge emission analyzerto constantly maintain the stored energy of the capacitor immediatelybefore the discharging operation to enhance the accuracy andreproducibility of the analysis, it is difficult for the conventionalsystem to maintain the stored energy at a constant level. The presentinvention has been developed to solve this problem, and its objective isto provide an emission analyzer in which the stability of the storedenergy of the discharge capacitor for generating a spark discharge isimproved to stabilize the emission of an objective element associatedwith the discharge.

Means for Solving the Problem

The present invention aimed at solving the previously described problemis an emission analyzer having a discharge electrode to be positionedleaving a predetermined gap to a sample, an igniter circuit connected tothe discharge electrode, a capacitor for storing electrical energy to beused for discharging, and a capacitor-charging circuit for charging thecapacitor, which is characterized by including:

-   -   a) a direct-current power source;    -   b) a flyback transformer with a primary winding connected to the        direct-current power source and a secondary winding for        supplying electrical energy to the capacitor;    -   c) a switching device for controlling the on/off state of an        excitation current supplied from the direct-current power source        to the primary winding of the flyback transformer;    -   d) a current detector for detecting the current value of the        excitation current; and    -   e) a controller for controlling the operation of the switching        device to repeat an on/off operation a predetermined number of        times at predetermined intervals of time, the on/off operation        including the steps of turning on the switching device and then        turning off the switching device when the current value detected        by the current detector has reached a predetermined value.

In the emission analyzer according to the present invention, thecontroller turns on the switching device to supply an excitation currentto the primary winding of the flyback transformer, and then turns offthe switching device to suspend the current supply when the excitationcurrent has reached a predetermined fixed value. The excitation energythat will be accumulated in the flyback transformer depends on theinductance of the winding and the value of the current flowing throughthe winding. Therefore, by discontinuing the current supply when thecurrent value has reached the predetermined value, it is possible tohave the same amount of excitation energy accumulated in the flybacktransformer within each on/off cycle of the switching device, regardlessof the voltage of the direct-current power source. The controllercharges the capacitor (i.e. transfers the excitation energy, which hasbeen accumulated in the flyback transformer, to the capacitor) byrepeating the on/off operation of the switching device a predeterminednumber of times. Therefore, the amount of energy held in the capacitorat the moment of discontinuing the charging operation will be constant.

The period of time from the beginning to the completion of charging isconstant since both the number of repetitions of the on/off operation ofthe switching element and the interval of time (i.e. the time requiredfor one on/off cycle of the switching element) are definitely set. Ifthe period of time from the completion of charging to the initiation ofthe operation of the igniter circuit is constant, then the period oftime from the beginning of charging to the initiation of the operationof the igniter circuit becomes constant, and the consumption (or loss)of energy due to, for example, the circuit for detecting the chargedvoltage of the capacitor also becomes constant. Therefore, the amount ofenergy stored in the capacitor immediately before the generation ofspark discharge by the igniter circuit will be constant.

The amount of energy stored in the capacitor at the moment ofdischarging depends on the number of repetitions of the on/off operationof the switching device. Therefore, it is preferable that the number ofrepetitions can be externally set. It is also preferable that thepredetermined value to be used as a reference for the current value ofthe excitation current to turn off the switching element can beexternally set.

Effect of the Invention

In the emission analyzer according to the present invention, the amountof energy stored in the discharge capacitor at the moment of generatinga spark discharge becomes constant without being affected by a voltagechange of the DC power source, a capacitance change of the capacitor orother factors. As a result, the generation of plasma by the dischargewill be performed in a stable manner and under virtually the sameconditions, and the emission of an objective element within the plasmawill also be stabilized. Thus, the emission analysis can be performedwith higher levels of accuracy and reproducibility.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram mainly showing the emitting section of anemission analyzer according to an embodiment of the present invention.

FIG. 2 is a principal timing chart in the emitting section of theemission analyzer according to the embodiment.

FIG. 3 is a block diagram showing the emitting section of a conventionalemission analyzer.

FIG. 4 is a waveform diagram in the main components of the emittingsection of the conventional emission analyzer.

FIG. 5 is an illustration showing a variance of the charged voltage inthe emitting section of the conventional emission analyzer.

EXPLANATION OF NUMERALS

1 Capacitor-Charging Circuit

11 DC Power Source

12 Flyback Transformer

13 Switching Element

15 Excitation Current Detector

16 Charge Controller

2 Capacitor Circuit

21 Rectifying Diode

22 Discharge Capacitor

23 Charged Voltage Detector

3 Igniter Circuit

31 Igniter Transformer

32 Igniter Driver

4 Emission Stand

41 Discharge Electrode

42 Metallic Sample

5 Photometric Section

BEST MODE FOR CARRYING OUT THE INVENTION

One embodiment of the emission analyzer according to the presentinvention is hereinafter described with reference to the drawings. FIG.1 is a block diagram mainly showing the emitting section of the emissionanalyzer according to the present embodiment, and FIG. 2 is itsprincipal timing chart. The components identical to those of theconventional emission analyzer shown in FIG. 3 are denoted by the samenumerals and hence will not be described in detail.

In the emitting section of the emission analyzer according to thepresent embodiment, the capacitor-charging circuit 1 has an excitationcurrent detector 15 serially connected to the primary winding of theflyback transformer 12 and the switching element 13. The current valuedetected by the excitation current detector 15 is fed to the chargecontroller 16. The charge controller 16 determines the timing of theon/off operation of the switching element 13 with reference to thecurrent detection value received from the excitation current detector 15instead of the voltage value detected by the charged voltage detector23. In the case of FIG. 1, the charged voltage detector 23 is employedfor different purposes; for example, it is used to check whether or notthe discharge capacitor 22 is correctly charged, prevent the capacitorfrom being overcharged, or detect the complete discharging of thecapacitor.

In the emitting section, the following operations are consecutivelyperformed: When the switching element 13 is turned on by the chargecontroller 16, a DC excitation current is supplied from the DC powersource 11 to the primary winding of the flyback transformer 12, toaccumulate excitation energy in the flyback transformer 12. As shown inFIG. 2, the excitation current almost linearly increases during the “on”period of the switching element 13. The value of this excitation currentis continually detected by the excitation current detector 15, while thecharge controller 16 determines whether or not the current value hasreached a predetermined threshold level i1. If the excitation currenthas been determined to have reached the threshold level i1, the chargecontroller 16 immediately turns off the switching element 13, wherebythe excitation current supply is discontinued. When the switchingelement 13 is turned off, a counter electromotive force arises in thesecondary winding of the flyback transformer 12. Due to this force, theexcitation energy that has been accumulated in the flyback transformer12 is supplied into the discharge capacitor 22. Thus, the dischargecapacitor 22 is charged.

In the charge controller 16, the length of time for one on/off cycle ofthe switching element 13 (i.e. the intervals of time from one turn-onaction of the switching element 13 to the next) and the number ofrepetitions of the on/off operation are previously set. Accordingly, thecharge controller 16 repeats the on/off operation of the switchingelement 13 the preset number of times at the preset intervals of timebefore discontinuing the charging operation.

A change in the voltage of the DC power source 11 changes the upwardgradient of the excitation current during the “on” period of theswitching element 13. For example, the upward gradient of the excitationcurrent in phase (B) in FIG. 2 is smaller than that in phase (A) sincethe voltage of the DC power source 11 is lower in phase (B). Regardlessof the gradient, the switching element 13 is maintained in the “on”state until the excitation current reaches the threshold level i1.Therefore, the “on” period of the switching element 13 does not alwayshave the same length; a smaller upward gradient of the excitationcurrent leads to a longer “on” period of the switching element 13. Thisincrease in the length of the “on” period is accompanied by acorresponding decrease in the length of the “off” period since, asstated earlier, the length of time for one on/off cycle is definitelyset (i.e. unchanged).

For an inductance L of the winding and excitation current I, theexcitation energy E that will be accumulated in the flyback transformer12 is indicated by:

E=(½)·L·I ².

This equation states that the amount of excitation energy will beconstant if the excitation current I is constant. Therefore, the sameamount of excitation energy will be transferred from the flybacktransformer 12 to the discharge capacitor 22 every time the switchingelement 13 is turned off. Since the number of on/off operations of theswitching element 13 is definitely set, the discharge capacitor 22 willhold the same amount of stored energy when the charging operation isdiscontinued.

When, as described previously, the charging of the discharge capacitor22 is completed, the igniter transformer 31 in the igniter circuit 3under the command of a control circuit (not shown) generates a highvoltage from the igniter driver 32 to produce a spark discharge betweenthe discharge electrode 41 and the metallic sample 42 of the emissionstand 4. The spark discharge vaporizes the atoms of the elements presenton the surface of the metallic sample 42, and emissions of lightoriginating from those elements occur within the plasma created betweenthe discharge electrode 41 and the metallic sample 42.

Since the length of time for one on/off cycle of the switching element13 and the number of repetitions thereof are constant, (these values aremaintained at least throughout the analysis of one sample), the periodof time from the beginning to the completion of charging is constant.Meanwhile, the electric charge held by the discharge capacitor 22 leaksto the charged voltage detector 23, which is connected to the dischargecapacitor 22 in parallel, and a bleeder resistance (not shown), which isalso connected to the discharge capacitor 22 in parallel for the sake ofsecurity. These elements consume a small portion of the storedelectrical energy. However, the amount of this energy consumption alsobecomes constant since the aforementioned period of time is constant.Furthermore, since the length of time from the beginning of charging tothe execution of discharging, i.e. to the initiation of the operation ofthe igniter circuit 3, is also constant, the discharge capacitor 22 willalways hold the same amount of electrical energy when the dischargingoperation is performed.

In this manner, the same conditions can be constantly created for thespark discharge and emission within the plasma between the dischargeelectrode 41 and the metallic sample 42 in the case of periodicallyrepeating the spark discharge. Thus, unfavorable variances of thephotometrical values are suppressed to enhance the accuracy andreproducibility of the analysis.

From the preceding description, it is evident that changing the numberof repetitions of the on/off operation of the switching element 13 willresult in a different amount of energy stored in the discharge capacitor22 when the discharging operation is performed. Accordingly, it ispreferable to provide a means for changing the number of repetitions ofthe on/off operation if the discharge conditions should desirably bechanged according to the sample type, a variance of the dischargeatmosphere or other factors.

The previous embodiment is a mere example of the present invention; anymodification, addition or correction that is appropriately made withinthe spirit of the present invention will evidently fall within the scopeof the claims of this patent application.

1. An emission analyzer having a discharge electrode to be positionedleaving a predetermined gap to a sample, an igniter circuit connected tothe discharge electrode, a capacitor for storing electrical energy to beused for discharging, and a capacitor-charging circuit for charging thecapacitor, which is characterized by comprising: a) a direct-currentpower source; b) a flyback transformer with a primary winding connectedto the direct-current power source and a secondary winding for supplyingelectrical energy to the capacitor; c) a switching device forcontrolling an on/off state of an excitation current supplied from thedirect-current power source to the primary winding of the flybacktransformer; d) a current detector for detecting a current value of theexcitation current; and e) a controller for controlling the operation ofthe switching device to repeat an on/off operation a predeterminednumber of times at predetermined intervals of time, the on/off operationincluding steps of turning on the switching device and then turning offthe switching device when the current value detected by the currentdetector has reached a predetermined level.
 2. The emission analyzeraccording to claim 1, wherein a number of repetitions of the on/offoperation of the switching device can be externally set.
 3. The emissionanalyzer according to claim 1, wherein the predetermined value to beused as a reference for the current value of the excitation current toturn off the switching element can be externally set.
 4. The emissionanalyzer according to claim 2, wherein the predetermined value to beused as a reference for the current value of the excitation current toturn off the switching element can be externally set.